@Diane, crewel wool is two ply wool. Technically, hand embroidery is pulling stuff through stuff with a needle so you can use any wool you like and any fabric. In reality the fabric needs to be sturdy, 'proper' embroidery people say to use linen with crewel but I have used linen/polycotton mix and it works fine.
For other embroidery you can use anything! I back mine with muslin to strengthen it and help keep its shape in the hoop or frame
PS I want to be Trish Burr when I grow up! Her stuff is like what I want to be able to do but I am not good enough!